There was no real possibility of effective French action against Germany during the Polish campaign. This is largely due to the swiftness of the Polish defeat. German victory was quite clear after two weeks and Polish defeat was made certain on September 17th when the Soviets invaded. The last Polish city to offer resistance, Gdynia, surrendered on October 2nd. By this time, some German forces in Poland were already being sent west to the Rhine.
The BEF didn't begin deployment until war broke out and wasn't in position until October. So any attack upon Germany during the Polish campaign would have been carried out by France alone. Violating the neutrality of any of the Low Countries was politically impossible. Failing this, there were only two options:
(1) A Rhine crossing between Karlsruhe and the Swiss border.
(2) An attack into the 100 mile-long border region between the Moselle and the Rhine. This region was protected by the prepared defenses of the Siegfried line, including small concrete works and minefields.
Both options presented serious military problems, so the German force of 11 regular and 32 reserve divisions on the western border was enough to prevent the French from making any quick breakthrough.
Furthermore, French planners did not consider large-scale offensive operations possible until about 16 days into a war. By this time, Poland was clearly doomed and France found it necessary to assume a defensive posture.
The 4th French Army did, in fact, cross into the Moselle-Rhine triangle on the 8th but avoided the Siegfried line and any serious combat. A larger attack was called off on the 22nd because Polish defeat was clear.
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